Verizon has confirmed that its mobile hotspot option for the iPhone 4 will be available for an extra $20 per month, according to MacWorld. The “personal hotspot” will allow up to five Wi-Fi-ready devices to share an iPhone 4’s 3G data connection.

Verizon also announced that the hotspot feature will come with an extra 2GB a month, which will be entirely separate from the basic data plan. But be forewarned: if you go over the 2GB limit, every extra gigabyte will cost an additional $20.

AT&T currently supports iPhone tethering, but only for a single device tethered through a USB or Bluetooth connection. AT&T also charges $20 a month for the feature, but doesn’t include any additional gigs to cover tethering — all data used from tethering counts against a customer’s basic data plan. It’s suspected that Apple may include the five-device hotspot feature as part of its iOS 4.3 upgrade.

Verizon’s chief operating officer Lowell McAdam has also added a caveat to his announcement on Tuesday that his company will offer an unlimited data plan for the iPhone 4: unlimited data plans won’t be offered indefinitely. McAdam has now said Verizon will eventually drop unlimited plans and move to adopt a tiered data structure in the “not too distant future.”

AT&T offered an unlimited data option for the iPhone before switching to a tiered data structure last year. Users who had existing unlimited data plans were able to retain their contracts and are being grandfathered in to AT&T’s new data structure. It’s not clear if Verizon will adopt a similar approach once the company drops its unlimited data option.